Voltage-gated sodium channels (VGSCs) are essential for action potential generation. Regulation of voltage- gated ion channel function is an important pathway by which neuronal signaling and brain function is regulated, and G-protein coupled receptors (GPCRs) form a major element of the endogenous transduction mechanisms by which this occurs. However, unlike other ion channels, VGSCs have been believed to be relatively insensitive to modulation by GPCR signaling. We have recently identified a pathway that is modulated by agents known to interact with the GPCR CB1 (cannabinoid receptor). This pathway is widespread, present in the vast majority of neocortical neurons, and strong enough to completely and reversibly block VGSC currents when maximally stimulated. This novel, dynamic signaling pathway is positioned to substantially modulate neuronal excitability and brain function. Detailed knowledge about the underlying mechanisms is crucial to understand its many effects. These preliminary findings may fundamentally change our understanding of the mechanism of action of endocannabinoids. The objectives of this proposal are to determine how endocannabinoids regulate VGSCs. We will complete this undertaking by studying VGSC function using patch- clamp methods and live cell imaging in neurons in acute neocortical brain slices, following acute isolation, and in primary cultures. We will employ mouse models. We are ideally suited to perform this project because of our preliminary data and expertise. Successful completion of these specific aims will characterize the mechanism of action of inhibition of sodium channels by this novel pathway and characterize a new mechanism by which endocannabinoids can affect neuroplasticity. Our rationale is that the identification and characterization of a novel and prevalent receptor(s) and downstream pathway will facilitate our understanding of a prevalent and potentially powerful neurobiological signaling pathway. Elucidation of the pathway will provide a detailed characterization of a new drug target that may be relevant to a wide range of diseases characterized by unbalanced excitability.